Process for the vaporisation of liquefied low-boiling gases



May 24, 1960 B. RIEDIGER 2,937,504

PROCESS FOR THE VAPORISATION OF LIQUEFIED LOW-BOILING GASES Filed Oct.10, 1956 2 Sheets-Sheet 2 20C lat/n. gauge Methane 2,937,504 7 PROCESSFOR THE VAPORISATION F LIQUEFIED LOW-BOILING GASES Bruno Riediger,Frankfurt am Main, Germany,"assignor to Metallgesellschaft A.G.,Frankfurt am Main, German Filed Oct. 10, 1956, s... No. 615,1ss I Claimspriority, application Germany Oct. 10, 1955 7 Claims. (Cl. 62 -53) Stats Pa ent 10 In the production of natural gas as well as in theproduction and processing of petroleum, large quantities of methane,ethane and similar low-boiling hydrocarbons, or gases rich in suchsubstances, are produced. Since such' substances cannot always be put togood use in the areas where they are produced, they are frequently pipedto distant consumers. These possibilities ofutilisation, however, are inmany cases, e.g. the petroleum sources of the Middle or Far East, theGulf Coast or Venezuela, not present, or only present to a limitedextent. The transport of natural gas or the like in very long trunksupply pipes also involves considerable costs. w

Another possibility for taking methane, ethane, or similar gases fromareas where there is a surplus and conveying them to consumer areas liesin the liquefaction of the gases and the despatch of these liquids inships,

or other large-capacity containers. It is necessary, here,

that the temperatures employed for the liquefied gases should be belowtheir critical temperature.

.It is only in this way that the liquefied gases can be stored ortransported under the usual pressures for transport receptacles-wherepossible at atmospheric pressure or at a pressure a little aboveatmospheric. If the critical. temperature were to be exceeded, completeevaporation would take place, and the pressure would rise to such anextent that the walls of containers which can be economically producedwould not be able to withstand the same.

In the areas of surplus there is a cheap supply of natural gas orproducts from petroleum processing. The liquefaction of the gases whichhave to be transported is, therefore, not subject to any high costsbecause the not inconsiderable energy requirements for this process canbe met without much expenditure.

The present invention aims at recovering a major part of the energywhich has been expended in the liquefaction of low-boiling substancessuch as natural gas, methane,

ethane, or the like, at the place of destination after-the liquefiedsubstances have been transported in largecapacity-containers, especiallyships. To this end, according to the invention, the vaporis'ation orheating of the liquefied substances is carried out in conjunction with acyclic process in which a working medium is cooled and then heatedagain. It is effective ifthe liquefied substances, are utilised ascooling agents, preferably at fairly low temperatures, in one or morecyclic processes, e.g. inone of these in which a vaporous mediumwhich'has done work is completely or partially condensed by cooling, thecondensate formed from the vapour being subsequently vaporised again. Asan example acondensate is vapourised, the vapours of which aresupp1ied,'undei' the pressure generated by the evaporation,

. 2,937,504 Patented lVlay 24, I960.

pressure gradients, and possibly, before being utilised for thegeneration of energy, they can be heated further and their enthalpyraised, by the application of heat, to such a value that the gases,after expansion in the engine, are available with the requiredconditions (pressure and temperature) so that they are ready for furtheruse.

When use is being made of the cold content of the liquefied gases in acyclic process it is desirable to choose the working medium of thecyclic process so that the heat required simply for re-vaporisation canbe extracted from the surroundings, e.g. the atmosphere or supplied fromthe heat content of water. Waste energy can also bemade available forthis purpose. The cyclic process is, thus,

so arranged that the temperatures involved are for the greater partbelow the ambient temperature.

It is also possible to use the liquefied substances in succession in twoor more cyclic processes, by so selecting the working media in thesuccessive cyclic processes that they are condensed at temperatureswhich are higher from one cyclic process to the next. If, for example,it is desired to utilise the cold content of liquefied ethane which hasbeen transported in ships in the liquid state at atmospheric pressure,this is available at the site of utilisation at a temperature themaximum of which is approximately- -88 C. In view of the need for a mostfavourable pressure gradient in the cyclic process it appears advisableto choose as working medium for this the hydrocarbon next above ethaneas regards boiling, point viz. propane.

The condensation temperature for the propane will then lie, for example,above approximately -40 C. if, for reasons which will be understood, itis desired to avoid a vacuum in the system. The propane can then bevaporised at temperatures up to +96 C. and then superheated. Thepressure to be applied is arbitrary and can i also be above the criticalpressure. Thus there is an even greater enthalpy gradient available forthe generation of energy.

In the case of liquefied methane the transport temperature atatmospheric pressure lies around -160 C. For

its utilisation at the destination it is expedient to use 7 cold ofliquefied ethane;

Fig. 2 represents the utilisation of the cold of liquefied methane inthe same manner; and

. Fig. 3 is a diagrammatic exemplification of another embodiment of theinvention in which the liquefied gas itself is also utilised in anenergy generating plant as the working medium.

In the process, represented in the diagram of Fig. 1,. the liquefiedethane passes at a temperature of -88,, C. and under a pressure ofatmospheres gauge, through a conduit 17 into the condenser 1 where itserves forjthe; condensation of propane vapour coming through a couduit12 from an expansion engine 2 at a temperature of 25 C. and under apressure of 2 atmospheres absolute. In the condenser 1 the propanevapour is condensed; the condensate flows through conduit 13 to apump 5which brings it to a pressure of about 65 atm. absolute and leads itthrough a conduit 14 to a vaporiser 3. The propane vapour which is drawnoff from this vaporiser through conduit 15 is raised to a temperature ofabout 125 C. in the superheater 4 and passes under a pressure of about60 atm. absolute through conduit 16 to the expansion engine 2. Thispreferably takes the form of a turbine for driving a generator 17 forelectrical energy. After expansion, the propane starts its cycle afreshin the manner described. The ethane is heated in the condenser to about-40 C. and led through conduit 18 to the point at which it will befurther exploited or utilised.

In the vaporiser 3 the heating agent in the first place may be air drawnfrom the ambient atmosphere but it must be absolutely dry so as to avoidthe formation of ice on the heat-exchange surfaces. As soon as thepropane has been heated to such an extent that the wall temperatures inthe vaporiser are above C., the further heating can also be effected bymeans of waste energy, e.g. in the form of hot water from the condensersof steam turbines or the cooling water system of diesel engines or wastesteam under low pressure. The heating agent enters the vaporiser throughconduit 19 and leaves itv through conduit 20.

For the pressures and temperatures in the propane cycle other values maynaturally also be chosen. It is desirable to choose them so that on onehand the lowest pressure in the cycle remains above the externalatmospheric pressure, to prevent air penetrating in the event of a leakoccurring, and on the other hand the working pressure is only chosen sohigh that, not only common design may be applied for the apparatus forthe cyclic process but also an adequate enthalpy gradient can be madeavailable in the expansion engine.

In the embodiment illustrated in Fig. 1, the highest working pressure inthe propane cycle is given as 60 atm. absolute and the highesttemperature 125 C. There is no reason why even higher working pressuresshould not be employed. In the last resort the criterion for a suitablechoice of the maximum working pressure is the amount of the throughput,so that the blades in the first stage of the turbine have lengths withwhich favourable turbine efficiencies can be reached.

It is not necessary to avoid exceeding the critical conditions in thepropane cyclic process, as is proved by the successful application ofthe Benson Principle in the construction of steam power plant, butallowance must be made for this condition in the construction of thevaporiser 3 and the superheater 4.

It is advisable so to select the maximum temperature in the cyclicprocess that, with the usual efficiencies of the expansion engines, thevapour issuing therefrom is just in the saturated state. However, anydeviations from such a selection have only an insignificantly harmfuleffect,

on the economic efficiency obtainable. Other conditions being the same,a reduction in temperature at the outlet from the superheater 4 resultsin the wet steam range being reached at the outlet from the expansionengine 2, which in the case of turbines, for example, can lead toerosion of the blades. A rise in temperature has a similar efiect on theefficiency of the cyclic process as such, as does the use of superheatedsteam in the usual power plant process. It is advisable, however, ingeneral to fix the maximum temperature so that waste heat can beutilised in the vaporiser 3 and in the superheater 4. The feed pump canalso consist of two units if a storage tank for the circulating fluidhas to be interposed between the condenser and the vaporiser, one ofthese units being located before the reservoir and one after it.

An example of the method for exploitation in stages of the energygradient in case of liquefied methane em- 4 ploys ethane in a primarycycle and propane in the secondary cycle as the working medium, as shownin Fig. 2. The choice of pressures and temperatures is governed by thesame considerations as those prevailing in the method of operationdescribed with reference to Fig. 1.

Liquid methane passes through a conduit 31 at a temperature of about l60 C. and under a pressure of about 90 atm. gauge into a condenser 21. Itflows out from said condenser 21 through a conduit 42 into a condenser26, which it enters at about 100 C. and leaves at a temperature of -40C. through conduit 43 to be conducted to the point of its furtherexploitation or utilisation. The pressure drop of the methane during itspassage through the condensers amounts to about 10 atm. absolute, sothat the vaporised methane in the conduit 43 is under a pressure ofabout atm. gauge.

In the primary cycle I the working medium is ethane, and in thesecondary cycle H propane is used. Both cycles can be designed on thediagram of the cycle shown in Fig. 1 and in the same manner. Forexample, the primary cycle is provided with an expansion engine 22together with the plant 37 for producing electrical energy, the feedpump 25, the vaporiser 23 and the superheater 24, and the cycle II withthe same equipment 27, 41, 26, 10, 28 and 29, which are connected bymeans of the conduits 32, 33, 34, 35, 36 and 52, 53, 54, 55 and 56,respectively. As an example, the ethane in cycle I enters the expansionengine 22 at a temperature of 80 C. and a pressure of 60 atm. absoluteand leaves it under a pressure of 2 atm. absolute and a temperature of75 C. The condensate passes at the same temperature from the condenser21 to the feed pump 25 in which its pressure is raised to approximately65 atm. absolute. The propane cycle II shown in Fig. 2 can work with thesame pressures as the cycle in Fig. 1 and corresponding temperatures.

With the operations according to the invention, the low temperature of aliquid hydrocarbon or mixture of hydrocarbons or of substances withsimilar physical properties can be utilised for producing energy, in away which is particularly favourable from both the economical andtechnical standpoints, from amounts of heat which shall be drawn off ata low temperature and which represent either a desired coolingperformance or a waste energy which cannot be utilised readily. Theliquefied gas is thereby vaporised and brought to a higher tem perature.

If the temperature desired for the vaporised methane, etc. is near theambient temperature and higher than can be attained by the examplesdescribed with reference to Figs. 1 and 2, this can be achieved, forexample, by raising the pressure in the cyclic system and hence thetemperatures in the condensers. Alternatively it can be effected by theaddition of a third, for example an analogous cycle, for butane, forexample, thus obtaining an outlet temperature for the gas leaving thecondenser of, for example, +5 C.

According to Figs. 1 and 2 the liquefied gas is passed through thecondensers under fairly high pressure. Since, initially, it is liquidwhich has to be conveyed, this conveyance under pressure only requireslow energy rates. This conveyance under pressure can be utilised withadvantage for the further recovery of energy from the liquefied andre-vaporised gas. It can also be of advantage in other cases, however,for example, when it is required to shift the start of vaporisation ofthe liquefied gas to higher temperature ranges. This again has theadvantage that the flow cross sections in one part of the installationcan be kept smaller and the heat transfer coefiicients which are morefavourable with liquids can be utilised in a wider temperature range.

In Fig. 3 a pump 62 which is provided inside a ship 61 or other storagecontainer pumps the liquefied gas first through an installation K whichmay take the form illustrated in Fig. 1 or 2 and in which the liquefiedgas is vaporised and heated. From this installation K the gas passesthrough a conduit 63, for example with a temperature of a little over 0C. and under a pressure of 80 atm. gauge, into the heater, for examplethe tubular heater 64 or into an apparatus of similar action. In

this heater the gas is brought, by means of external heat, to such atemperature that, after expansion in an engine 65, it reaches a desiredcondition, making it suitable, for example, for its further conveyancethrough a distribution system to the consumers. The inlet temperature tothe heater 64 should preferably be chosen a little above freezing pointso as to avoid icing of the tube system by the steam contained in theflue gases. The choice of this temperature point is more or lessarbitrary, however, and has no great influence on the resultsobtainable. The higher the temperature point due to the supply of heatin the preceding part of the installation K, the less heat will benecessary to supply to the heater 64. If, for example, at the outletfrom the energy generating plant 65, the gas is to be available at 20C., 1 atm. gauge, it is advisable that, with a pressure of 60 atm. gaugebefore the expansion engine 65, it should have a temperature of about200 C. There is nothing against the use of an even higher pressure. Inthis case a corresponding increase in the temperature now given as 200C. as a result of the expansion process should take place. The otherpressures employed in the form of embodiment according to Fig. 3 can beobtained as approximations from the anticipated pressure drop and can becalculated for each case individually according to the known technicalrules.

The efiiciency of this production of energy from heat energy is veryfavourable as compared with the usual power plant processes because theheat which has to be supplied to the tubular heater 64 is simply theheat for superheating. To this extent it coincides with the gas turbineprocess. However, the considerable consumption of energy for thenecessary work of compression, which markedly reduces the overallefficiency of the gas turbine process, is eliminated because, accordingto the invention, the working medium being in the liquid state can bebrought to the necessary pressure with a much smaller energy input. Asregards the last mentioned point it coincides with the steam process.

The process of the present invention has the great advantage over thelatter, however, in that no vaporisation heat needs to be supplied tothe working medium in the tubular heater since this heat can bewithdrawn from the surrounding air in the preceding processes or can bemade available from waste heat.

Iclaim:

1. A method of recovering energy from liquefied low boiling gases whichcomprises vaporising such liquefied low boiling gases in heat exchangerelationship with a gaseous working medium which is condensed duringsuch vaporisation, raising the pressure of the thus condensed workingmedium to a higher pressure, then vaporising said condensed workingmedium at said higher pressure, expanding said vaporised working mediumto recover energy and then recycling the vaporized working medium to thecondensation step where it is recondensed to provide a cyclic process.

2. The process of claim 1 in which the vaporisation of said low boilinggases is carried out under pressure.

3. The process of claim 1 in which at least two working media areprovided, each Working medium being sequentially condensed, the pressureof the condensed working medium raised, the condensed working mediumvaporised under the raised pressure, the resulting vapors; expanded torecover energy therefrom and then recon densed in a separate cyclicprocess and in which the liquefied gases are vaporised by passing themsuccessively as cooling agents in heat exchange relationship with theworking medium of each cyclic process to effect the condensation of theworking medium in each cyclic process.

4. The process of claim 3 in which ethane is employed as the workingmedium of the first cyclic process and a hydrocarbon having a higherboiling point than the hydrocarbon of the next preceding cyclic processis employed in each successive cyclic process.

5. The process of claim 1 in which the vaporisation of .the condensedworking medium is carried out below the ambient temperature.

6. The process of claim 1 in which the vaporised gas obtained byvaporisation of the liquefied gas in heat exchange relationship with theworking medium is expanded to a predetermined pressure and temperatureto recover energy therefrom.

7. The process of claim 1 in' which additional heat is supplied to thevaporised gas obtained by vaporisation of the liquefied gas in heatexchange relationship with the working medium and such heated gas isthen expanded to recover energy therefrom.

References Cited in the file of this patent UNITED STATES PATENTS FranceSept. 26,

